Naphthenic acids are usually defined as a monobasic carboxyl group attached to a saturated cycloaliphatic structure. The molecular formula is given by CNH2N+IO2 where I is −2 for monocyclic acids and −4 for bicyclic acids. It has been accepted in the oil industry that almost all organic acids in crude oil are called naphthenic acids. Naphthenic acids in crude oils are mixtures of low to high molecular weights varying from approximately 100 to greater than 1300 units.
Naphthenic acids are recognized for their corrosive behavior and as an initiator of fouling, emulsifying and other undesired reaction during production and at the refinery stages. Some organic acids are thought to arise from the biodegradation process. This process has a large impact on oil viscosity and thus on oil flow rate and ultimately the economic gains from the oil production. The extent of biodegradation in oil columns is highly variable depending on many factors such as temperature, geologic history, distance to the oil-water contact etc.
It is thus seen as desirable to have the ability to measure or estimate the presence and concentration of naphthenic acids in reservoirs fluids. Early knowledge of the concentration of naphthenic acid can be used, for example, in field development plans which integrate the technical and economic aspects of drilling production wells and installing production facilities.
The current methods for assessing or the screening of oil samples for naphthenic acid as reflected, for example, in U.S. Pat. No. 6,281,328 to Satori et al., U.S. Pat. No. 7,160,728 to Chimenti et al., or published U.S. Patent Application No. 2007/0298505 involve either a titration method (TAN) or various spectroscopic methods, including mass spectroscopy (MS), infrared spectroscopy (IR), ultra-violet spectroscopy (UV), or nuclear magnetic resonance (NMR). As described in these and other sources, including for example, “Simple Method to Determine Partition Coefficient of Naphthenic Acid in Oil/Water” by Anders Bitsch-Larsen and Simon Ivar Andersen, to be published in the Journal of Chemical and Engineering Data, IR methods are usually based on measuring the absorption or reflectance at 1708 cm-1 (carboxylic or C═O band) or, to a lesser degree at 1728 cm-1 or 1637 cm-1. The methods as described are typically performed in a laboratory under ambient conditions.
It is further known to analyze formation fluid in the borehole using downhole analyzing tools such as the CFA™ of Schlumberger. The CFA is described, for example, in the Oilfield Review, Autumn 2003 issue, pp. 54-61, in co-owned U.S. Pat. Nos. 6,437,326 to Yamate and Mullins and 6,768,105 to Mullins et al., and a similar type of downhole analysis tool is described in the U.S. Pat. No. 7,362,422 to DiFoggio and Bergren. Known downhole instruments are designed to be carried downhole on a tool string such as the Schlumberger's MDT™ and are able to analyze fluid flow through the tool in the visible and near-infrared range of the electromagnetic spectrum.
In view of the known art, it is seen as one object of the invention to provide a method and apparatus for determining the presence and/or concentration of naphthenic acids in formation fluids at a downhole location.